fed to the converter unit is developed across a resistor (R-1)
in the junction box, Figure 15, which is in series with these meters.
Identification markers may be injected during a survey by the
oircuit consisting of battery B-1, potentiometer R-9 and timer T-1l.
The input from the detector is disconnected and a voltage is supplied
to the grid of the balanced amplifier to drive the output signal to
full scale.
The marker is initiated by an external push button.
Its
duration is determined by timer T-1 and its amplitude by R-9.
30202
Converter
The converter section turns the D.C. voltage developed across
R-1 into amplitude modulated pulses.
A small part of the full-wave rectified 60 cps line voltage
appears across R-25 end is amplified by V-2B.
The output of V-2B
triggers a univibrator producingconstant amplitude square waves
(Figure lja) at a 120 pps rate,
converter tube V-2A.
These are fed to the grid of the upper
The signal from the bridge circuit is fed into
‘the grid of tube V-3, which is in series with the cathode of V-2A.
The detector signal determines the current through both V-3 and V-2A
and hence the gain of V-2A.
Therefore, the amplitude of the
pulses
appearing across R-1]1 (Fig. lb) are proportional to the signal from
the bridge.
The pulses across R-11 are transformer coupled to the
comparator unit,
Batteries, B-2 and 43, and resistors, R-12, 13 and 14, fix the
bias of V-3 so that the tube operates over a linear portion of its
characteristic.
A floating ground is used since the signal from the
bridge is at a potential above chassis ground.
R-26 and C-8, 9 and
10 provide a filter from the floating ground bus to chassis ground.
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